Various forms of molding materials comprising a continuous reinforcing fiber bundle and a thermoplastic resin as a matrix are known; for example, thermoplastic prepregs, yarn, and glass mats (GMT). Such molding materials are characterized in that they are easy to mold because of the properties of thermoplastic resin; they are free from the burden of storage unlike thermosetting resins; and the resulting molded article have high toughness, so that it is excellent in recyclability. In particular, pelleted molding materials can be applied to molding methods that are excellent in economic efficiency and productivity such as injection molding and stamping molding, and are useful as industrial materials.
However, for impregnating a thermoplastic resin into a continuous reinforcing fiber bundle during the process for producing a molding material, such molding materials are disadvantageous in terms of economic efficiency and productivity, and therefore are not so widely used at present. For example, it is well known that impregnation of a resin into a reinforcing fiber bundle becomes difficult as the melt viscosity of the resin increases. In particular, thermoplastic resins having excellent dynamic properties such as toughness and ductility are high-molecular-weight, have a high viscosity compared to those of thermosetting resins, and require a higher process temperature. Thus, such thermoplastic resins have been unsuitable for producing a molding material easily with high productivity.
When a low-molecular-weight, i.e., low-viscosity thermoplastic resin is used as a matrix resin because of ease of impregnation, there is a problem in that the resulting molded article will have significantly decreased dynamic properties.
Further, as fiber-reinforced composite materials come to be used in harsher environments, higher heat resistance have been required for a matrix resin.
Under such circumstances, the presence of a low-melting-point and low-molecular-weight thermoplastic resin was not preferred because it caused deformation of a molded article under high-temperature conditions. Consequently, molding materials comprising a thermoplastic resin excellent in impregnation properties and heat resistance have been demanded.
Fiber-reinforced composite materials comprising a reinforcing fiber and a matrix resin are lightweight, can provide excellent strength properties, and can be designed to have any strength by controlling their fiber orientation. Therefore, such fiber-reinforced composite materials are widely used, for example, in sports applications such as golf shafts and fishing rods, aerospace applications such as aircraft parts and artificial satellite parts, and general industrial applications such as automobiles, marine vessels, electrical and electronic equipment housings, robot parts, windmills, tanks, bathtubs, and helmets. In producing a fiber-reinforced composite material, production methods in which prepregs used as intermediate substrates are laminated to form a laminate, the prepregs being obtained by impregnating reinforcing fibers with a matrix resin, are widely used because, in general, high fiber content is easily achieved and handling is relatively easy. As a matrix resin to be impregnated into reinforcing fibers in a prepreg, thermosetting resins such as unsaturated polyester resins, vinyl ester resins, and epoxy resins are often used because of the ease of impregnation into a fiber bundle, but the thermosetting resins become an insoluble and infusible polymer having a three-dimensional network structure via curing. Such a polymer is difficult to recycle, and a disposal problem becomes more serious.
As a thermoplastic matrix resin used for a prepreg, various resins such as polyethylene, polyester, polyamide, and polycarbonate are used. In applications that require high performance, such as aerospace applications, polyether ether ketone, polyetherimide, polyphenylene sulfide, and the like that are excellent in heat resistance, chemical resistance, and mechanical properties are suitably used.
However, such a thermoplastic resin prepreg has problems in that, in a production process for impregnating a fiber bundle with a matrix resin, a high temperature and a high pressure are required because of its high molecular weight compared to those of thermosetting resins; it is difficult to produce a prepreg having high fiber content; and a prepreg produced has so many unimpregnated parts that sufficient mechanical properties cannot be provided.
Fiber-reinforced composite materials comprising a continuous reinforcing fiber substrate and a matrix resin are lightweight and have excellent dynamic properties, and they are widely used in sports equipment applications, aerospace applications, general industrial applications, and the like. In particular, composite materials comprising a carbon fiber as a reinforcing fiber (CFRP) have a specific strength and specific rigidity that are superior to those of metal materials, and the amount thereof used is increasing mainly in aerospace applications. As a matrix resin, thermosetting resins have hitherto been preferably used because of their satisfactory impregnation into a reinforcing fiber substrate. Thermoplastic resins have been unsuitable for producing a molding material easily with high productivity because they are high-molecular-weight, have a high viscosity compared to those of thermosetting resins, and require a higher process temperature.
However, composite materials comprising a thermoplastic resin as a matrix resin have received attention in recent years for reasons that such composite materials can be molded in a short time; the resulting molded article is recyclable; and they are excellent in post-processability such as thermal adhesion and thermal reformation.
Further, fiber-reinforced composite materials comprising a reinforcing fiber and a matrix resin allows material design taking advantages of the reinforcing fiber and the matrix resin, and, consequently, their use is expanding to the aerospace field, transport equipment/industrial machine field, civil engineering and construction field, sports/leisure field, and the like.
As a reinforcing fiber, glass fiber, aramid fiber, carbon fiber, boron fiber, and the like are used. As a matrix resin, both thermosetting resin and thermoplastic resin are used, but thermosetting resin, which readily impregnates into a reinforcing fiber, is often used. However, fiber-reinforced composite materials comprising a thermosetting resin have problems in that the productivity is low because a long time is required for heat curing and that the pot life of a prepreg is restricted.
In contrast, fiber-reinforced composite materials comprising a thermoplastic resin as a matrix have been put to practical use for reasons that the productivity is high because such fiber-reinforced composite materials do not need curing reaction and that they are easily welded, repaired, and recycled.